RESUMEN
Here, we have studied the crystalline structure of bulk ZnX (X = O, S, Se, Te) and ZnF2 systems as a first step to understand the structures like ZnX and Zn-based systems like ZnO/ZnF2 interfaces, which are of utmost importance for possible technological applications. In addition, an adequate methodological description based on density functional theory (DFT) calculations is necessary. It is well known that plain DFT calculations based on local or semilocal exchange-correlation functionals fail to describe the correct band gap energy for these systems, whereas nonlocal approaches, such as hybrid-based functionals, can compensate the underestimation of band gap. To contribute to the assessment, DFT studies were performed within semilocal Perdew-Burke-Ernzerhof (PBE) and two nonlocal functionals, hybrid Heyd-Scuseria-Ernzerhof (HSE) and PBE + U functionals. Our results confirm that PBE underestimates the energy band gap values, from 33.0 to 42.8% for ZnX compounds compared to the experimental values. Applying the hybrid HSE functional, we obtained a band gap dependency in relation to the range of separation of the nonlocal exact exchange, in general decreasing the band gap error and improving the lattice constant description. In addition, using the PBE + U approach, we have investigated the localization of the Zn d-states and its effect on the band gap in ZnX and ZnF2. We found an increase in the band gap with increasing Hubbard parameter, which introduces on-site Coulomb corrections for the Zn 3d states. In the same context, the relevance to include the Hubbard corrections for the O 2p states (and X p states) is highlighted. Thus, considering PBE + U, the error in ZnO band gap, for example, decreases to 5.1%, in relation to the experimental value. Finally, ZnO-12L/ZnF2-4L superlattices are found to exhibit conventional electronic properties, such as low fundamental band gap, smaller than either of the parent materials. Our first-principles calculations reveal that the unexpected band gap reduction is induced by the conducting layers that tend to penetrate the interface and decrease the band gap, leading to the transport of carriers through the interface to ZnF2, which, even with a high band gap for charge transfer, can be interesting for photovoltaic applications.
RESUMEN
The engineering of semiconductor materials for the development of solar cells is of great importance today. Two topics are considered to be of critical importance for the efficiency of Grätzel-type solar cells, the efficiency of charge separation and the efficiency of charge carrier transfer. Thus, one research focus is the combination of semiconductor materials with the aim of reducing charge recombination, which occurs by spatial charge separation. From an experimental point of view, the combining of materials can be achieved by decorating a core with a shell of another material resulting in a core-shell system, which allows control of the desired photoelectronic properties. In this context, a computational simulation is mandatory for the atomistic understanding of possible semiconductor combinations and for the prediction of their properties. Considering the construction of ZnO/ZnX (X = S, Se or Te) interfaces, we seek to investigate the electronic influence of the shell (ZnX) on the core (ZnO) and, consequently, find out which of the interfaces would present the appropriate properties for (Grätzel-type) solar cell applications. To perform this study, we have employed density functional theory (DFT) calculations, considering the Perdew-Burke-Ernzerhof (PBE) functional. However, it is well-known that plain DFT fails to describe strong electronic correlated materials where, in general, an underestimation of the band gap is obtained. Thus, to obtain the correct description of the electronic properties, a Hubbard correction was employed, i.e. PBE+U calculations. The PBE+U methodology provided the correct electronic structure properties for bulk ZnO in good agreement with experimental values (99.4%). The ZnO/ZnX interfaces were built and were composed of six ZnO layers and two ZnX layers, which represents the decoration process. The core-shell band gap was 2.2 eV for ZnO/ZnS, â¼1.71 eV for ZnO/ZnSe and â¼0.95 eV for ZnO/ZnTe, which also exhibited a type-II band alignment. Bader charge analysis showed an accumulation of charges in the 6th layer of ZnO for the three ZnO/ZnX interfaces. On the basis of these results, we have proposed that ZnO/ZnS and ZnO/ZnSe core-shell structures can be applied as good candidates (with better efficiency) for photovoltaic devices.